1. Field of the Invention
The present invention relates to a method for modelling fluid flows in a fractured medium crossed by relatively large fractures, as specified hereafter.
2. Description of the Prior Art
In a reservoir under production, the pressure and flow rate conditions imposed for the producing and injection wells induce flows of the fluids in place in the reservoir (oil, gas and water). Simulation of these flows determines the evolution of the pressures and of the saturations in the reservoir in the course of time. Because of the very heterogeneous nature of fractured reservoirs, the fluids move relatively fast through the network of fractures and much more slowly in the matrix. Simulation of flows in the context of a fractured reservoir therefore requires a very good representation of the major heterogeneities formed by the fractures. The accuracy of this representation depends on the type of simulation used and on the size of the fractures to be modelled. Thus, very precise simulation of a well test requires representation of the exact geometry of the fractures network. Conversely, taking into account a dense network of small fractures on the scale of a field will involve an equivalent simplified representation.
Currently, the petroleum industry notably uses double-porosity (double medium) models for simulating flows in fractured mediums, which are not applied to the real geologic medium in all its complexity but to a homogenized representation, according to the reservoir model referred to as double medium model described for example by Warren and Root in “The Behavior of Naturally Fractured Reservoirs”, SPE Journal, 1963. Any elementary volume of the fractured reservoir is considered to be two equivalent homogeneous mediums on the scale of the simulator grid cell: a fracture medium and a matrix medium. On the scale of the reservoir, the fluids flow mainly through the fracture medium and fluid exchanges occur locally between the fractures and the matrix blocks. This representation, which does not show the complexity of the network of fractures in a reservoir, is however efficient at the level of a reservoir grid cell whose dimensions are typically 100 m×100 m. Double medium modelling allows reproduction of the flow behavior of the two mediums and their interactions without requiring explicit modelling of the two mediums.
French Patent 2,757,947 and corresponding U.S. Pat. No. 6,023,656 filed by the assignee describe a technique for determining the equivalent fracture permeability of a network of fractures in an underground multi-layer medium from a known representation of this network. This technique allows systematical connection of fractured reservoir characterization models to double-porosity simulators in order to obtain a more realistic modelling of a fractured underground geologic structure.
French Patent 2,757,957 filed by the assignee describes a technique allowing a simplified modelling of a porous heterogeneous geologic medium (such as a reservoir crossed by an irregular network of fractures for example) in form of a transposed or equivalent medium, so that the transposed medium is equivalent to the original medium, in relation to a determined type of physical transfer function (known for the transposed medium).
French Patent application 98/15,727 filed by the assignee also describes a method for modelling fluid flows in a fractured multi-layer porous medium by taking into account the real geometry of the network of fractures and the local exchanges between the porous matrix and the fractures at each node of the network. The fractured medium is discretized by means of a grid, the fracture cells are centered on the nodes at the various intersections of the fractures, each node being associated with a matrix volume, and the flows between each fracture cell and the associated matrix volume are determined in a pseudosteady state.
There are cases where the previous techniques are difficult to implement, in the presence of an medium crossed by large fractures or sub-seismic faults whose hydraulic behavior cannot be homogenized on the scale of the cell. Explicit modelling of these objects is therefore necessary a priori, but their large number makes such an approach prohibitive on the scale of a field (too large number of cells and numerical constraints). The same problem arises for reservoirs containing thin and very permeable layers whose behavior is similar to that of large horizontal fractures.